PROJECT SUMMARY Neural circuits modulating metabolic state and reproduction must communicate with each other to maintain homeostasis. Despite an increasing prevalence of both metabolic and reproductive disorders, our understanding of neural circuitry linking energy homeostasis and reproduction remains rudimentary. Melanocortin 3 receptor (MC3R) is ideally positioned, both anatomically and functionally, to mediate direct communication between reproductive and metabolic circuits. MC3R is expressed in multiple areas of the brain, including the anteroventral periventricular nucleus (AVPV) and the arcuate nucleus of the hypothalamus (ARH). The AVPV and ARH are critical sites controlling reproduction and energy balance, and they are linked by direct neural connections. Therefore, MC3R-expressing neurons in the AVPV and ARH may represent important components of a core neural circuit that functions to integrate the neural control of energy balance and reproductive state. However, the cellular identity of MC3R-expressing neurons in the AVPV and ARH has not been determined, nor has the organization of their neural projections been defined. Recent evidence suggests MC3R may also functionally mediate the exchange of information between metabolic and reproductive state. During metabolic challenges resulting from reproductive state, such as pregnancy and ovariectomy, deletion of MC3R results in mice gaining too little weight or too much weight, respectively. Deletion of MC3R results in additional reproductive and metabolic disturbances in males and females, including adverse responses to fasting, disruptions to feeding behavior, and altered circulating hormone concentrations. Moreover, females with MC3R mutations typically exhibit more aberrant reproductive and metabolic phenotypes, compared with those observed in mutant males. However, a detailed understanding of the mechanisms causing these differential responses in males and females is lacking. Because sex steroids specify the organization of sexually dimorphic neural circuits during critical periods of development, it is possible that exposure to estrogen during these periods may cause permanent changes in the architecture of MC3R regulated circuits, and consequently, reproductive and metabolic physiology. The overall hypothesis of this application is that MC3R neurons provide a neurological substrate for integration of metabolic signals with reproductive status, and that this circuit may be configured differently in males and females. As a first step toward testing this hypothesis, the following two specific aims will be pursued: 1) Define the molecular phenotypes of neurons that express MC3R in the AVPV and ARH of male and female mice, and map the organization of their neural projections; 2) Determine if the organization and function of MC3R circuits depends upon estrogen receptor signaling during postnatal development. Completion of these aims will establish a novel framework for understanding neurological mechanisms underlying how MC3R coordinates metabolic regulation with reproductive state.